to cut flowers.
Figure 10.30: Cytokinins promote cell division and are necessary for growing plants in tissue
culture; a small piece of a plant is placed in sterile conditions to regenerate a new plant. ( 15 )
Abscisic Acidis misnamed because it was once believed to play a role in abscission (the
dropping of leaves, fruits and flowers), but we now know abscission is regulated by ethylene.
The actual role of abscisic acid is to close the stomata and maintain dormancy. When
a plant is stressed due to lack of water, abscisic acid signals the stomata to close. This
prevents excess water loss through the stomata. When conditions are not ideal for a seed
to germinate or for a winter bud to put out its leaves, abscisic acid signals for dormancy
to continue. When conditions improve, the levels of abscisic acid drop and the levels of
gibberellins increase, signaling that is time to break dormancy (Figure10.31).
Auxinsare hormones that influence many different processes in plants. Auxins produced at
the tip of the plant are involved in apical dominance, preventing the growth of side branches.
Inapical dominancethe main central stem of the plant is dominant over other side stems;
the main stem grows more strongly than other stems and branches. When the tip of the
plant is removed, the auxins are no longer present and the side branches begin to grow. This
is why pruning generally will help produce a fuller plant with more branches. Auxins are
also involved in tropisms, which will be discussed in the next section.
Tropisms
Plants may not be able to move, but they are able to change their growth in response to a
stimulus. Growth toward or away from a stimulus is known as atropism.The ability of a
plant to curve its growth in one direction is achieved with the signaling of auxin. The auxin
moves to one side of the stem, where it starts a chain of events that elongate the cells on